The outer membrane proteins of selected C. trachomatis serotypes are being studied with the goal of utilizing purified surface protein antigens for the development of improved immunodiagnostic assays for human chlamydial diseases. Surface radioiodination has identified the major outer membrane protein (MOMP) of C. trachomatis. Serologically related serotypes share a common MOMP with respect to subunit molecular weight. The MOMP from the L2/434 and C serotypes have been isolated and purified to homogeneity. Studies are currently being done to describe the antigenic properties of the purified MOMP from these serotypes and to evaluate their potential as chlamydial serodiagnostic test antigens.